This invention relates to a method and apparatus for inspecting a test lens, and more particularly to a method and apparatus f or determining an optical quality characteristic such as the modulation transfer function (MTF) of a test lens. Although the apparatus and method are usable for inspecting virtually any test lens, they are particularly adapted for the inspection of monofocal and multifocal intraocular lenses.
Virtually any lens has imperfections which cause an image formed by that lens to vary from the ideal. In order to assure that any imperfections in a test lens are within allowable tolerances, it is important to carefully inspect the lens. Lens inspection is particularly important f or lenses and lens systems where high performance is required. Intraocular lenses, which are adapted to be implanted in the human eye to replace the natural lens of the eye, are one important example of a high performance lens.
One important test for a lens is determining MTF of the lens. Generally MTF is a measure of the efficiency with which a lens or optical system is able to contrast regions of a target at various spatial frequencies. When the modulation transfer function drops to a predetermined level, it is presumed that the image can no longer be resolved, and the corresponding spatial frequency becomes the resolution limit of the system. MTF also provides a quantitative description of contrast and accounts for the effects of both diffraction and different types of aberrations on image quality. MTF is considered to be a more useful measure of lens performance than resolution alone.
MTF can be determined using either indirect or direct techniques. In the indirect method, images of a slit or pinhole are used and complex mathematics are employed to determine MTF. One such indirect system is shown by way of example in Klingman U.S. Pat. No. 3,938,892.
In direct MTF measurement systems, light is directed through a target to a test lens which forms an image of the target. For example, the target may he a grating, in which event the image is of the bars and spaces of the grating. Alternatively, the grating may be replaced with a target having sinusoidally varying light transmissive properties as disclosed, for example, in Kuperman U.S. Pat. No. 4,653,909. In any event, the image formed by the test lens has alternate light and dark regions which enable contrast to be measured at the different spatial frequencies defined by the widths of the light and dark regions of the image. Examples of direct MTF measurement systems are shown in Weiser U.S. Pat. No. 3,743,427 and Holly U.S. Pat. No. 3,930,732.
In order to obtain information on image contrast along different meridians of the test lens, it is known to rotate the target to move the regions of higher and lower spatial frequencies to different meridians. By taking contrast readings at different meridians, modulation transfer function can be calculated. One problem with this system is that the rotation of the target to provide different spatial frequencies along various meridians of the test lens introduces inaccuracies in the measurement needed to determine MTF. This is because these measurements are very sensitive to even very small changes in optical system location.